Cold fusion refers to a proposed nuclear fusion process of unknown mechanism offered to explain a group of disputed experimental results first reported by electrochemists Martin Fleischmann and Stanley Pons. It is sometimes termed "Low Energy Nuclear Reaction" (LENR) in contrast to the more common fusion reactions such as those inside stars and high energy experiments. The field originates with reports of an experiment by Martin Fleischmann, then one of the world's leading electrochemists,[1] and Stanley Pons in March of 1989 where they reported anomalous heat production ("excess heat") of a magnitude they asserted would defy explanation except in terms of nuclear processes. They further reported measuring small amounts of nuclear reaction byproducts, including neutrons and tritium.[2] The small tabletop experiment involved electrolysis of heavy water on the surface of a palladium (Pd) electrode.[3]

The media reported that nuclear fusion was happening inside the electrolysis cells,[4] and these reports raised hopes of a cheap and abundant source of energy.[5] Hopes fell when replication failures were weighed in view of several reasons cold fusion is not likely to occur, the discovery of possible sources of experimental error, and finally the discovery that Fleischmann and Pons had not actually detected nuclear reaction byproducts.[6] By late 1989, most scientists considered cold fusion claims dead,[7] and cold fusion subsequently gained a reputation as pathological science.[8] However, a small community of researches continues to investigate cold fusion[7][9][10][11] claiming to replicate Fleishmann and Pons' results including nuclear reaction byproducts.[12][13] These claims are largely disbelieved in the mainstream scientific community.[14]

In 1989, the majority of a review panel organized by the US Department of Energy (DOE) found that the evidence for the discovery of a new nuclear process was not persuasive. A second DOE review, convened in 2004 to look at new research, reached conclusions similar to the first.[15]